In Japan, only 30% of the primary energy supply is used as effective energy, with about 70% being eventually lost to the atmosphere as heat. The heat generated by combustion in industrial plants, garbage-incineration facilities or the like is lost to the atmosphere without conversion into other energy. In this way, a vast amount of thermal energy is wastefully discarded, while acquiring only a small amount of energy by combustion of fossil fuels or other means.
To increase the proportion of energy to be utilized, the thermal energy currently lost to the atmosphere should be effectively used. For this purpose, thermoelectric conversion, which directly converts thermal energy to electrical energy, is an effective means. Such a thermoelectric conversion utilizes the Seebeck effect, and is an energy conversion method for generating electricity in which a difference in electric potential is produced by creating a difference in temperature between both ends of a thermoelectric material.
In such a method for generating electricity utilizing thermoelectric conversion, i.e., thermoelectric generation, electricity is generated simply by setting one end of a thermoelectric material at a location heated to a high temperature by waste heat, and the other end in the atmosphere (room temperature) and connecting conductive wires to both ends. This method entirely eliminates the need for moving parts such as the motors or turbines generally required for electric power generation. As a consequence, the method is economical and can be carried out without generating gases by combustion. Moreover, the method can continuously generate electricity until the thermoelectric material has deteriorated.
Therefore, thermoelectric generation is expected to play a role in the resolution of future energy problems. To realize thermoelectric generation, large amounts of a thermoelectric material that has a high thermoelectric conversion efficiency and excellent heat resistance, chemical durability, etc. are required.
Used as an index of thermoelectric conversion performance of materials is the dimensionless figure of merit (ZT) of a thermoelectric material given by a calculation based on the following equation:ZT=S2T/ρκ                wherein S is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity; and κ is the thermal conductivity. It is known that the higher the figure of merit value, the higher the conversion efficiency.        
As can be seen from the above formula, a high Seebeck coefficient S is effective for increasing the figure of merit (ZT). It is advantageous to lower other factors, such as the electrical resistivity and thermal conductivity.
CoO2-based layered oxides, such as Bi2Sr2Co2O9, etc., have been previously reported as p-type thermoelectric materials that can be used at high temperatures in air (e.g., Japanese Patent Nos. 3069701, 3089301, and 3472814; Japanese Unexamined Patent Publication Nos. 2001-223393 and 2003-282964; and International Publication No. WO 03/000605). As materials capable of achieving further excellent thermoelectric conversion performance, materials have been demanded which has a low electrical resistivity and a low thermal conductivity as well as a high Seebeck coefficient.